Configuring an IPsec tunnel between Openswan and Windows 2000/XP with x509

This document describes how to get Openswan working with various other IPSec stacks, including Openswan and Windows 2000/XP. If you have any difficulties with this process, please e-mail the Openswan mailing list, or if you can’t get help from there, e-mail me at: ipsec@natecarlson.com. If you are using clients which benefit from receiving an IP address on the remote network (Windows, PocketPC, etc), you may want to consider L2TP over IPSec instead of the method described below. Jacco de Leeuw’s pages cover this in a good amount of detail; I also have a basic walkthrough available at my L2TP-over-IPsec page.

If you’re not sure if IPSec is right for you, I have written a quick document about some of the various types of VPN available under Linux. It is available at: http://www.natecarlson.com/linux/linux-vpn.php. I hope this helps clear up some questions.

IMPORTANT NOTE: On March 1, 2004, the FreeS/WAN maintainers announced that the FreeS/WAN project is ending, for many reasons. The Openswan project is going to be taking over development. Openswan is based on Super FreeS/WAN, and already includes most of the patches that people wanted. I’ve updated these directions to use examples for Openswan 2.1.2; they should still run as-is on FreeS/WAN 2.0 with the X.509 patches, and will work with FreeS/WAN 1.99+X.509 and Openswan 1 with some minor modifications. They should also work as-is with Strongswan. I no longer cover patching FreeS/WAN with X.509; if you are going to start with a base FreeS/WAN installation, you will need to follow the directions at http://www.strongsec.com/freeswan on how to patch it.

IMPORTANT NOTE #2: As of June 17 2004, this document has been updated to reflect Openswan configuration instead of FreeS/WAN. I’ve also reorganized a few things; hopefully it will flow better now. Please let me know if you run into any problems with the new configuration. If you need it, the old page is available at: http://www.natecarlson.com/linux/ipsec-x509-fs1.php.

NOTE #3: Not nearly as important as above, but just wanted to note that I do occasionally post notes about new VPN options and such on my blog; see the VPN category at: http://www.natecarlson.com/category/geek-stuff/vpn. Also, if you are interested in consulting services to help you set things up, I am available on a very limited basis – please see my consulting page.

For the sake of this document, I’m assuming you want to use X.509 certificates for authentication. It is possible to use RSA keys or pre-shared keys, but I find the X.509 method to be the most scalable and easiest to maintain for a decent-sized user base. I am also assuming that you will need your own Certificate Authority dedicated to VPN usage – if you already have access to a CA, you may just want to generate certificates from there (if that’s the case, you can just skim this section.) If you need more details that I am going into here, please read the OpenSSL documentation — it’s fairly detailed. For CA certificate management, my examples use the utilities included with OpenSSL itself – there are third-party tools out there that make this a bit simpler, but I want to keep dependencies low. Note that you do not necessarily need to use your Openswan gateway as the Certificate Authority – it can be any box with OpenSSL installed. In fact, it may be better to use a different box, so if an attacker gains access to your Openswan gateway they don’t have access to your CA, too. If you have any suggestions on how to make this process simpler, please let me know!

Now, on to the good stuff – let’s start setting up our own CA.

1) Find your openssl.cnf file. This file has default values for OpenSSL certificate generation. Here’s a few locations for various distributions:

Debian: /etc/ssl/openssl.cnf
RedHat 7.x+: /usr/share/ssl/openssl.cnf

Open this file in your favorite editor. We will need to change the following options:

‘default_days’: This is the length of time, in days, that your certificates will be valid for, and defaults to 365 days, or 1 year. I recommend setting this to ‘3650’, as that will give you 10 years of validity on your certificates. Since this is for internal use, I am ok with the security ramifications of having a certificate valid for a long time – if you lose it or whatnot, you can revoke it without a problem.

‘[ req_distinguished_name ]’ section: You don’t really *need* to change the options below req_distinguished_name; they just set the default options (such as location, company name, etc) for certificate generation. I find it’s easier to set them here than re-type them for every certificate.

2) Create a directory to house your CA. I generally use something like /var/sslca; you can really use whatever you want. Change the permissions of the directory to 700, so that people will not be able to access the private keys who aren’t supposed to.

3) Find the command ‘CA.sh’ (some distributions rename it to just ‘CA’; don’t ask me why.) Locations on various distributions:

Debian: /usr/lib/ssl/misc/CA.sh
RedHat 7.x+: /usr/share/ssl/misc/CA

Edit this file, and change the line that says ‘DAYS=”days 365″‘ to a very high number (this sets how long the certificate authority’s certificate is valid.) Be sure that this number is higher than the number is Step 1; or else Windows may not accept your certificates. Note that if this number is too high, it can cause problems – I generally set it for 15-20 years.

4) Run the command ‘CA.sh -newca’. Follow the prompts, as below. Example input is in red, and my comments are in blue. Be sure to not use any non-alphanumeric characters, such as dashes, commas, plus signs, etc. These characters may make things more difficult for you.

nate@example:~/sslca$ /usr/lib/ssl/misc/CA.sh -newca
CA certificate filename (or enter to create) (enter)
Making CA certificate ...
Using configuration from /usr/lib/ssl/openssl.cnf
Generating a 1024 bit RSA private key
.............................................................................+++
........................................+++
writing new private key to './demoCA/private/./cakey.pem'
Enter PEM pass phrase:(enter password --this is the password you will need to create any other certificates.)
Verifying password - Enter PEM pass phrase: (repeat password)
-----
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]: (Enter your country code here)
State or Province Name (full name) [Some-State]: (Enter your state/province here)
Locality Name (eg, city) []: (Enter your city here)
Organization Name (eg, company) [Internet Widgits Pty Ltd]: (Enter your company name here, or leave blank)
Organizational Unit Name (eg, section) []: (OU, if you like. I usually leave it blank.)
Common Name (eg, YOUR name) []: (The name of your Certificate Authority)
Email Address []:(E-Mail Address)
nate@example:~/sslca$

Let’s also generate a crl file, which you’ll need on your gateway boxes:

nate@example:~/sslca$ openssl ca -gencrl -out crl.pem

You’ll need to update this CRL file any time you revoke a certificate.

That’s it, you now have your own certificate authority that you can use to generate certificates.

You will need to generate a certificate for every machine that will be making an IPSec connection. This includes the gateway host, and each of your client machines. This section details how to create the certificate, and convert it to formats needed for Windows and such.

Again, we’ll be using the CA.sh script. Except this time, instead of telling it to create a new Certificate Authority, we’re telling it to request, then sign a certificate:

Common Name (eg, YOUR name) []:host.example.com(enter)This can be a hostname, a real name, an e-mail address, or whatever

Email Address []:user@example.com(enter) (optional)

Please enter the following ‘extra’ attributes

to be sent with your certificate request

A challenge password []:(enter)

An optional company name []:(enter)

Request (and private key) is in newreq.pem

What we just did is generate a Certificate Request – this is the same type of request that you would send to Thawte or Verisign to get a generally-accepted SSL certificate. For our uses, however, we’ll sign it with our own CA:

nate@example:~/sslca$ /usr/lib/ssl/misc/CA.sh -sign

Using configuration from /usr/lib/ssl/openssl.cnf

Enter PEM pass phrase:(password you entered when creating the ca)

Check that the request matches the signature

Signature ok

The Subjects Distinguished Name is as follows

countryName :PRINTABLE:’US’

stateOrProvinceName :PRINTABLE:’State’

localityName :PRINTABLE:’City’

organizationName :PRINTABLE:’ExampleCo’

commonName :PRINTABLE:’host.example.com’

emailAddress :IA5STRING:’user@example.com’

Certificate is to be certified until Feb 13 16:28:40 2012 GMT (3650 days)

Sign the certificate? [y/n]:y(enter)

1 out of 1 certificate requests certified, commit? [y/n]y(enter)

Write out database with 1 new entries

Data Base Updated

(certificate snipped)

Signed certificate is in newcert.pem

Next, move the output files to names that make a bit more sense for future reference.

nate@example:~/sslca$ mv newcert.pem host.example.com.pem

nate@example:~/sslca$ mv newreq.pem host.example.com.key

That’s all that’s required for Openswan boxes – you’ll need these two files, along with the file ‘cacert.pem’ from the ‘demoCA’ directory, and the ‘crl.pem’ file you generated earlier.

If this certificate is needed for a Windows box, you’ll need to convert it to a p12 format:

You’ll need to install Openswan each Linux box you want to speak IPSec.

Openswan now integrates all of the important patches, including X.509 and NAT Traversal. If you want to build it from scratch, you can download it from http://www.openswan.org/code, and follow the installation directions included with the package.

You now have two options for which IPSec stack you want to install in the kernel – you can either use Openswan’s IPSec stack (Klips), or use the built-in IPSec stack in the 2.6 kernel (26sec). If you are running on a stock 2.4 kernel, the only option is Klips. You’ll need to patch NAT Traversal support into your kernel (if you intend to use it), and build the ipsec.o kernel module. Otherwise, if you are using a 2.6 kernel or a 2.4 kernel with backported 26sec support (such as the kernel Debian provides), you don’t need to touch the kernel-land at all – you can just install the Openswan user-land utilities and go. Note that there isn’t as of yet an option to use Klips on the 2.6 kernel; it is on the Openswan developer’s to-do list, but isn’t a real high priority.

You’ll also need the user-land utilities. If you are installing from source, ‘make programs ; make install’ should get you what you need. Otherwise, if you are running Debian testing or unstable, you can just run ‘apt-get install openswan’ to get the user level utilities. ATrpms provides a Openswan package for recent versions of RedHat and Fedora Core; for more information on that, see http://atrpms.net.

Once you’ve selected and set up your IPSec stack and installed the user-land programs, you’re ready to move on to configuring Openswan.

This discusses how to install the certificate on your gateway machine. These same steps apply for installing the cert on Openswan clients, too. I’m assuming you’ve already created a certificate for each machine (see the “Generating a Certificate” section) – if that’s not the case, please go back and do that now.

1) Install the files in their proper locations (if installing to a remote machine, please be sure to copy the files in a secure manner):

: RSA host.example.com.key “password”
The password above should be the PEM passphrase that you entered while generating the SSL certificate.

2) Configuring ipsec.conf

/etc/ipsec.conf should look something like the configuration below (note that the indentation is important; without it, openswan will fail):

version 2.0

config setup

interfaces=%defaultroute

nat_traversal=yes

virtual_private=%v4:10.0.0.0/8,%v4:172.16.0.0/12,%v4:192.168.0.0/16

conn %default

keyingtries=1

compress=yes

disablearrivalcheck=no

authby=rsasig

leftrsasigkey=%cert

rightrsasigkey=%cert

conn roadwarrior-net

leftsubnet=(your_subnet)/(your_netmask)

also=roadwarrior

conn roadwarrior

left=%defaultroute

leftcert=host.example.com.pem

right=%any

rightsubnet=vhost:%no,%priv

auto=add

pfs=yes

conn block

auto=ignore

conn private

auto=ignore

conn private-or-clear

auto=ignore

conn clear-or-private

auto=ignore

conn clear

auto=ignore

conn packetdefault

auto=ignore

This configuration will set things up so anyone with a valid certificate signed by your CA will be able to connect to your host. There are two connection profiles: one for a connection directly to the gateway, and one for the client to connect to the network behind the gateway. This configuration also includes NAT Traversal configuration that will allow anyone a host behind a NAT gateway using RFC1918 private addresses (defined in the ‘virtual_private’ line) to connect. All of the ‘auto=ignore’ entries are used to disable Opportunistic Encryption (OE), as it can cause problems if not configured properly.

If you are planning on having Windows boxes connect to your host using L2TP over IPSec, you’ll also need the following connections, somewhere above the ‘roadwarrior’ definition:

conn roadwarrior-l2tp

pfs=no

leftprotoport=17/0

rightprotoport=17/1701

also=roadwarrior

conn roadwarrior-l2tp-updatedwin

pfs=no

leftprotoport=17/1701

rightprotoport=17/1701

also=roadwarrior

In addition, if you want to have clients tunnel all traffic via IPSec, you’ll need a connection that allows that. The following is what I recommend (again, add above roadwarrior):

1) Follow the steps under ‘Generating a Certificate‘ to create a new certificate for the client machine, modifying file names and such as needed. (We will refer to the files for this client as ‘clienthost.example.com’.)

2) Copy the following files (in a secure fashion) over to your client:

host.example.com.pem (your gateway’s certificate file)

clienthost.example.com.key

clienthost.example.com.pem

cacert.pem

crl.pem

3) Copy the files into their proper locations:

$ cp clienthost.example.com.key /etc/ipsec.d/private

$ cp clienthost.example.com.pem /etc/ipsec.d/certs

$ cp host.example.com.pem /etc/ipsec.d/certs

$ cp crl.pem /etc/ipsec.d/crls

$ cp cacert.pem /etc/ipsec.d/cacerts/cacert.pem

4) Configure ipsec:

ipsec.secrets:

: RSA clienthost.example.com.key “password”

ipsec.conf:

version 2

config setup

interfaces=%defaultroute

nat_traversal=yes

conn %default

keyingtries=1

compress=yes

authby=rsasig

leftrsasigkey=%cert

rightrsasigkey=%cert

conn roadwarrior-net

leftsubnet=(your_subnet)/(your_netmask)

also=roadwarrior

conn roadwarrior

left=(ip.of.host)

leftcert=host.example.com.pem

right=%defaultroute

rightcert=clienthost.example.com.pem

auto=add

pfs=yes

conn block

auto=ignore

conn private

auto=ignore

conn private-or-clear

auto=ignore

conn clear-or-private

auto=ignore

conn clear

auto=ignore

conn packetdefault

auto=ignore

5) Start the VPN link, and make sure everything works:

# /etc/init.d/ipsec restart

$ ipsec auto –up roadwarrior

$ ipsec auto –up roadwarrior-net

6) If you would like to have the links start automatically, change ‘auto=add’ to ‘auto=start’.

NOTE: If you have previously installed SSH Sentinel, and want to use the built-in Windows IPSec stack, you will need to uninstall (or disable) SSH Sentinel, and enable the ‘ipsec’ service. I know this has tripped a few people up. This also applies for any other IPSec client you may have installed – you *need* to make sure it’s disabled before trying to use the built in IPSec service.

NOTE #2: The HTML guy at my previous employer went through and made screenshots of the process of importing a certificate. These screenshots are available at http://support.real-time.com/open-source/ipsec/index.html. Please do NOT e-mail Real Time with any questions related to this; I no longer work there, and don’t want them to get a flood of questions about this.

1) Create the certificate, again following the steps under ‘Generating a Certificate‘. We’ll assume that you call the Windows 2000 certificate ‘winhost.example.com’. You’ll need to follow the directions to output a .p12 file.

Also run the following, and make a note of it’s output:

$ openssl x509 -in demoCA/cacert.pem -noout -subject

You will need this for your VPN configuration.

2) Copy this file over to the Windows machine in a secure fashion, such as ‘scp’ or with a floppy disk. Don’t use FTP!

Edit your ipsec.conf (on the windows machine), replacing the “RightCA” with the output of the ‘openssl x509 -in demoCA/cacert.pem -noout -subject’; reformatted as below (you need to change the /’s to commas, and change the name of some of the fields — just follow the example below):

Now, ping your gateway host. It should say ‘Negotiating IP Security’ a few times, and then give you ping responses. Note that this may take a few tries; from a T1 hitting a VPN server on a cable modem, it usually takes 3-4 pings. Do the same for the internal network on the remote end, and you should be up!

I’ve tried to make it as simple as possible to follow the above instructions, but sometimes it just doesn’t quite work right. :) If you have trouble, feel free to e-mail me, or join the FreeS/WAN mailing list and ask your questions there (many times, you will get a quicker response there, as there are more people listening at any given time, and most of them are smarter than me!). But, just in case you’ve got one of the really common problems, here’s a few problems and solutions:

Yes, it is actually possible to enable logging on the Windows box! To do this, follow the directions at Microsoft’s Basic IPSec Troubleshooting in Windows 2000 page — look for the section entitled ‘Obtaining an Oakley Log’.

2) Pinging from the Windows side shows ‘Negotiating IP Security’, but the tunnel never comes up!

This is one of the most common problems people have, and is usually caused by problems with rightca= on the Windows side. To verify that you have that set properly, follow these instructions:

– Load the IPSec MMC you created earlier

– Click IP Security Policies; double-click on the FreeSwan tunnel

– Double-click roadwarrior-Host filter

– Click on the ‘Authentication Methods’ tab

– Click ‘Add’, then ‘Use a certificate from this CA’

– Click Browse, find your CA

– Copy/paste the text in the grayed-out box into your ipsec.conf

In many cases, that’ll clear up the issues – if it doesn’t, check your log for errors.

More troubleshooting tips to come soon, assuming I get time to write them. :)

Let me know if the above doesn’t make sense, and I’ll try to help you out. :)

just an aside, while doing this, i kept hitting this error during the last part of Step 4 under “setting up your certificate authority”
# openssl ca -gencrl -out crl.pem
Using configuration from /usr/lib/ssl/openssl.cnf
Enter pass phrase for ./demoCA/private/cakey.pem:
./demoCA/crlnumber: No such file or directory
error while loading CRL number
11493:error:02001002:system library:fopen:No such file or directory:bss_file.c:352:fopen(‘./demoCA/crlnumber’,’r’)
11493:error:20074002:BIO routines:FILE_CTRL:system lib:bss_file.c:354:

I have had the same problem today. I resolved it by commenting out the following line in my /etc/ssl/openssl.cnf:
crlnumber = $dir/crlnumber # the current crl number
The line is followed by a comment, that makes me think this is okay:
# must be commented out to leave a V1 CRL

which worked for me too. just FYI for anybody else following this guide.

Hi,I encountered a problem using your method. the following is the problem:
when I use the the following command: openssl pkcs12 -export -in winhost.example.com.pem -inkey winhost.example.com.key -certfile demoCA/cacert.pem -out winhost.example.com.p12, it said unable to load private key.
How can I solve the problem? Please help me, Thank you very much!